Many consumer and commercial devices require direct current (DC) power. Since alternating current (AC) power is readily available, power supply circuits which convert AC power to DC power are desirable.
A block diagram of a conventional power supply circuit 10 is depicted in FIG. 1. The power supply circuit consists of a voltage reducing device 11, rectifier 12, filter 13, and regulator 14. The voltage reducing device 11 steps the AC voltage down since DC-powered devices generally operate at a lower voltage (e.g., less than 12 volts) than commercially-supplied AC power (e.g., 120 volts). Next, the rectifier 12 converts the lower voltage level AC voltage to a pulsating DC voltage. The pulsating DC voltage is then filtered and regulated by the filter 13 and the regulator 14, respectively, to produce a relatively smooth DC voltage level.
FIG. 2 depicts a known power supply circuit 21 which embodies the functionality depicted in FIG. 1. The power supply circuit consists of a transformer 22, a full-wave bridge rectifier 23, and a capacitor 24. The transformer 22 steps down the input AC voltage to a usable level. The full-wave bridge rectifier 23, consisting of diodes D1-D4, converts the low level AC voltage into a pulsating DC voltage. The capacitor 24 filters and regulates the pulsating DC voltage to achieve a smooth DC output voltage level.
Although the conventional circuits such as the one described above have been used successfully, their design has significant shortcomings. For example, transformers tend to be heavy and bulky and thus unsuitable for miniaturized packaging. Transformers also tend to be relatively expensive given their inherent core material and windings requirements which are unlikely to be eliminated by technological advances. Aside from transformers, conventional circuits also tend to generate significant heat. Heat is generated not only by the normal operation of the circuit's components, but also by DC power generated by the circuit in excess of the load's requirements. To dissipate heat, the components are typically oversized which increases their cost and, again, poses problems in miniaturized packaging. Furthermore, as the temperature of the components rises, their operating characteristics tend to vary and their potential for failure increases.
Attempts have been undertaken to develop transformerless power supply circuits that are more efficient and compact. An example of such a circuit is depicted in FIG. 3. As shown, the circuit includes a rectifier in the form of a diode D1 and a current limiting resistor R1 at the AC input side of the power supply circuit. The circuit has a regulator in the form of a Zener diode D2 at the output side of the power supply circuit. As the AC input voltage rises, the output voltage supplied to a load increases until the output voltage exceeds the breakdown voltage of the Zener diode D2 (e.g. 5 Volts), causing the Zener diode D2 to conduct, thereby limiting the output current and voltage being supplied to the load.
In the power supply circuit of FIG. 3, excess power must be dissipated when the output voltage level is above the breakdown voltage of the Zener diode D2. The excess power is dissipated by the resistor R1, which must be physically large to dissipate the heat resulting from the large voltage drops across it. The power rating of the chosen resistor R1 will also limit the maximum AC input voltage that may be applied to the power supply circuit. For example, an input voltage of 120 Volts AC (VAC), with a Zener diode having a breakdown voltage of 5 Volts and an average current draw of 15 milliamps (ma), requires resistor R1 to have a minimum power rating of 1.73 Watts. An input voltage of 240 VAC, requires a resistor with a minimum power rating of 3.6 Watts. Thus, when the power rating of resistor R1 is chosen, the maximum level of AC input voltage is fixed.
Another known transformerless power supply circuit is disclosed in FIG. 4. As shown, the output side of the circuit contains a rectifier diode D1 leading to the load to ensure that only DC power is supplied to the load. The capacitor C1 is charged during positive portions of the AC cycle when the AC voltage is rising. During declining and negative portions of the AC cycle, the capacitor C1 supplies the power to the load.
Like the circuit of FIG. 3, however, the circuit disclosed in FIG. 4 requires physically large components to dissipate excess power due to varying levels of current being drawn by the load. For example, excess power must be dissipated when the output current at the output side of the power supply circuit exceeds the load current being drawn by the load. In the configuration shown, a large capacitor C1 and resistor R1 are required to handle the relativley large amount of power which must be dissipated. Furthermore, like the circuit of FIG. 3, the level of acceptable AC voltage is limited by the power ratings of the individual components.
Therefore, there is a need for a transformerless AC to DC power supply that is compact and efficient but avoids the need for oversized components to dissipate power. There is also a need for a power supply circuit that can accommodate different levels of AC input voltage without requiring its components to be changed out or oversized for the highest expected input power. The present invention fulfills these needs among others.